Magnesium oxide is a commercially important compound, being used in comparatively high purity for the production of refractories employed, for example, in the basic oxygen steel making process, and, in still higher purities, as a basic filler and for medicinal purposes. Historically, magnesium oxide has been produced by calcining magnesite, a naturally occurring magnesium carbonate, and, more recently, from magnesium hydroxide precipitated from sea water, brines, bitterns, and wastes from desalination plants.
Even for refractory uses, there is a need for increasingly pure magnesium oxide, particularly for specialty refractories used in basic oxygen steel making vessels.
While there are some natural magnesite deposits presently being worked, and yielding magnesium oxide which is extremely low in highly detrimental impurities such as boron oxide, relatively extreme beneficiation procedures are necessary to produce magnesium oxide of greater than about 96 percent* MgO content from such deposits. FNT *The terms "percent" and "parts" are used herein, and in the appended claims, to refer to percent and parts by weight, unless otherwise indicated.
The need of industry for magnesium oxide assaying greater than about 97 percent, and a substantial portion of the demands for less pure magnesium oxide, are supplied by precipitating magnesium oxide from sea water, brine, bitterns and waste from desalination plants. The precipitation is usually carried out by adding lightly burned dolomite to sea water or the like to bring the pH to a value on the basic side at which magnesium hydroxide is precipitated. Ultimately, after numerous processing steps, the precipitated magnesium hydroxide is recovered as an aqueous slurry which may contain as much as about 50 percent of magnesium hydroxide. Although magnesium hydroxide precipitated from sea water or the like can rather readily be recovered in sufficient purity to produce burned magnesium oxide containing up to about 98 percent of MgO, this process tends to produce a final product contaminated with undesirably high proportions of boron oxide, a highly undesirable constituent of magnesium oxide used as an insulating material with electrical heating elements and in producing magnesite refractories. While it has been found to be possible to reduce the boron oxide content of the final product, this can be done only by increasing to an undesirably high extent the content of at least one other impurity, by undesirably increasing the cost, or by a technique which is in some way detrimental or undesirable. Accordingly, there remains a need for an inexpensive way to increase the purity of magnesium oxide products, particularly those recovered by precipitation from sea water and the like.